Our long-range objective is to understand the mechanisms that limit spatial vision in humans with amblyopia. We propose to test a number of specific hypotheses and predictions about these factors, and to assess the limits and mechanisms of neural plasticity in adults and children with amblyopia. Aim 1. Crowding. Feature binding and positional uncertainty in amblyopic and peripheral vision: We hypothesize that both crowding and anomalous feature binding in peripheral and amblyopic vision can be explained by limits imposed by early processes in cortical area V1. We propose a series of experiments, using novel methods and modeling, to answer the following questions: i) Do feature binding and crowding share the same spatial properties? ii) Is there mis-mapping of retinal location to perceived visual space in amblyopia, and is this the result ofmis- wiring within feature maps or mis-registration between them? An Adaptive Optics Scanning Laser Ophthalmoscope allows us to deliver point stimuli to precisely identifiable single cones in the retina, and evaluate their perceived location iii)Does crowding impose a critical limit on reading in peripheral and amblyopic vision? Aim 2. Deficits exist at multiple levels in the amblyopic visual system: Recent results lead to the hypothesis that although dysfunction within the amblyopic visual system first occurs in area V1, it is amplified downstream. We will test the prediction that amblyopes will show deficits in "higher level" tasks that cannot be explained by low level considerations: i) second-order processing, ii) contour integration, and iii) temporal, spatial and/or capacity limits of attention. Aim 3. Neural Plasticity in normal and amblyopic vision: There is a surprising degree of neural plasticity in both normal and amblyopic adults as evidenced by perceptual learning, i) We will test the hypothesis that perceptual learning occurs at a high level and reflects the brain learning to attend to and use the most reliable information for the task, ii) Assess the time course, limits and mechanisms of plasticity in adults and young children with amblyopia undergoing both perceptual learning and clinical treatment. We predict that improvement following the successful treatment of amblyopia involves the same mechanisms that improve normal vision following perceptual learning, iii)We will test the prediction that the successful effects of treatment or perceptual learning in amblyopia will lead to increased foveal activation in the visual cortex, as evidenced by alterations in functional MRI.